1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a semiconductor device having a trench gate structure.
2. Background Art
Trench gate type MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are known as semiconductor devices suitable to power electronics and other applications requiring high withstand voltage and low ON resistance (see, e.g., JP 2002-083963A).
JP 2002-083963A discloses a MOSFET having a trench that is formed to reach an N-type substrate through a P-type well layer and an N-type drift layer. An N-type source layer is formed in the surface of the well layer. A buried electrode of polysilicon is formed via an insulating film in a region extending from the drift layer to the substrate in the trench. A gate electrode of polysilicon is formed via an insulating film in a region extending from the source layer through the well layer to the drift layer in the trench. The buried electrode is electrically insulated from the gate electrode.
In the configuration disclosed in JP 2002-083963A, the buried electrode is completely covered with the insulating film and does not form a p-n junction with the drift layer. Therefore, when the dopant concentration in the drift layer is increased for reducing ON resistance, it is difficult to completely deplete the drift layer and to obtain high withstand voltage.